System and methods for diagnosing soot accumulation on an exhaust gas recirculation valve

ABSTRACT

Methods and systems are provided for determining changes in a flow area of an exhaust gas recirculation (EGR) valve for EGR flow estimates due to soot accumulation on the EGR valve. In one example, a method includes indicating soot accumulation on the EGR valve based on a difference in EGR flow estimated with an intake oxygen sensor and with a pressure sensor coupled across the EGR valve. The determination of the difference of the EGR flow estimates may occur when the engine is not boosted.

FIELD

The present description relates generally to methods and systems for anexhaust gas recirculation system of an internal combustion engine.

BACKGROUND/SUMMARY

Engine systems may utilize recirculation of exhaust gas from an engineexhaust system to an engine intake system (intake passage), a processreferred to as exhaust gas recirculation (EGR), to reduce regulatedemissions and improve fuel economy. An EGR system, such as alow-pressure EGR system, may include various sensors to measure and/orcontrol the EGR. As one example, an engine intake system may include anintake gas constituent sensor, such as an oxygen sensor, which may beemployed during non-EGR conditions to determine the oxygen content offresh intake air. During EGR conditions, the sensor may be used to inferEGR based on a change in oxygen concentration due to addition of EGR asa diluent. One example of such an intake oxygen sensor is shown byMatsubara et al. in U.S. Pat. No. 6,742,379. However, the accuracy ofEGR estimates using the intake oxygen sensor may be reduced duringboosted engine operation and purge conditions when hydrocarbons areflowing through the intake system. EGR flow may then be estimated usingalternate EGR sensors. For example, the EGR system may also includedifferential pressure over valve (DP) sensor positioned around an EGRvalve for estimating EGR flow based on a pressure difference across theEGR valve and a flow area of the EGR valve. EGR flow estimates may thenbe used to adjust a position of the EGR valve and therefore adjust anamount of EGR provided to the engine.

As one example, a flow area of the EGR valve may change due to sootaccumulation, or other build-up, on the EGR valve. This change in EGRvalve flow area may impact the EGR flow estimate, and thus EGR control,based on measurements from a DPOV system including the DP sensor.However, the inventors herein have recognized that when the EGR valvecloses, an edge of the valve may cut into the valve seat and changes invalve lift due to soot build-up may not be detected. Thus, changes inthe EGR valve flow area affecting EGR flow may not be compensated forwith end-stop learning diagnostics using the DPOV position sensor,thereby resulting in inaccurate EGR flow estimates when using the DPOVsystem.

In one example, the issues described above may be addressed by a methodfor indicating soot accumulation on an exhaust gas recirculation (EGR)valve based on a difference in EGR flow estimated, during a firstcondition when the engine is not boosted, with an intake oxygen sensorand with a pressure sensor coupled across the EGR valve. In this way,changes in a flow area of the EGR valve due to soot accumulation may bedetected and subsequent EGR flow estimates may be corrected based on thesoot accumulation, thereby increasing an accuracy of EGR flow estimatesand resulting engine control.

As one example, a first EGR flow estimate may be determined using anintake oxygen sensor when the engine is not boosted and purge flow isoff. A second EGR flow estimate may be determined with a pressure sensorcoupled across the EGR valve, such as a differential pressure over valve(DP) sensor. The difference between the first and second EGR flowestimates may then be used to determine a change in flow area of the EGRvalve due to soot accumulation. More specifically, the second EGR flowestimate may be based on an output of the DP sensor and the flow area ofthe EGR valve, where the flow area of the EGR valve is estimated basedon a known cross-section of the EGR valve and an EGR valve positionbased on an output of an EGR valve position sensor. The change in EGRvalve flow area (due to soot build-up) may then be based on thedifference in the first and second EGR flow estimates, an expected EGRvalve flow area, and a first EGR flow estimated with the intake oxygensensor. The expected EGR valve flow area may be based on an output ofthe EGR valve position sensor and an EGR valve lift correction, wherethe EGR valve lift correction is learned during an EGR valve end stopand thermal compensation learning routine. In one example, the EGR valveends stop and thermal compensation learning routine may determine achange in flow are of the EGR valve due to a temperature differencebetween the stem and body of the EGR valve. In this way, changes in EGRvalve flow area affecting EGR flow estimates using the DPOV methoddescribed above may be determined and used to correct EGR flowestimates. In this way, more accurate EGR flow estimates may be used forengine control.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an example engine system including anintake oxygen sensor and exhaust gas recirculation system.

FIG. 2 is a flow chart of a method for estimating EGR flow with anintake oxygen sensor or differential pressure sensor based on engineoperating conditions.

FIG. 3 is a flow chart of a method for indicating soot accumulation onan EGR valve and determining a corrected EGR valve flow area based onsoot accumulation.

FIG. 4 is a graph illustrating changes in EGR flow estimates using anintake oxygen sensor and differential pressure sensor under varyingengine operating conditions.

FIG. 5 is a flow chart of a method for learning changes in EGR valveflow area due to changes in a temperature difference between an EGRvalve stem and body.

FIG. 6 is a flow chart of a method for learning a difference intemperature between an EGR valve stem and body at an EGR valve closingposition.

DETAILED DESCRIPTION

The following description relates to systems and methods for determiningchanges in a flow area of an exhaust gas recirculation (EGR) valve forEGR flow estimates. In one example, the changes in flow area of the EGRvalve may be due to soot accumulation on the EGR valve and/or a changein temperature difference between a stem and body of the EGR valve. Aturbocharged engine, as shown in FIG. 1, may include an intake oxygensensor located in an intake passage of the engine and a differentialpressure over valve (DP) sensor located in an EGR passage. The DP sensorand intake oxygen sensor may each be used to give estimates of an EGRflow through a low pressure EGR system. The EGR flow may be regulated byan EGR valve that, when open, may allow exhaust gas to recirculate to anintake passage from downstream of a turbine to upstream of a compressor.As shown in FIG. 4, EGR flow may be estimated using the intake oxygensensor and/or or the DP sensor based on engine operating conditions.When the EGR valve is open and a EGR is flowing through the EGR passage,a pressure differential across the EGR valve and the size of the openingof the EGR valve may be used to determine the magnitude of the EGR flow.A position sensor may be used to determine an EGR valve lift and thusestimate the area of the EGR valve opening, and the DP sensor mayprovide the differential pressure across the EGR valve. As described inFIGS. 5 and 6, the accuracy of the estimate of the EGR valve opening maybe increased by accounting for the thermal expansion of the EGR valve asa result of high EGR temperatures. Thus, taken together, measurementsfrom an EGR valve position sensor and a DP sensor may be used to providean estimate of the EGR mass air flow. Over time however, soot mayaccumulate on the EGR valve and decrease the effective flow area of thevalve opening. Without a method for estimating soot buildup, EGR flowestimates may become increasingly inaccurate as more and more sootaccumulates on the EGR valve.

As shown in FIG. 2, determining whether to use DPOV intake oxygen sensormeasurements (from a DPOV system including a DP sensor across an EGRvalve) for estimating EGR flow rates may be based on engine operatingparameters such as purge, boost, and intake mass air flow. The methoddescribed in FIG. 3 provides a technique for estimating soot buildup onthe EGR valve, and thus providing more accurate estimates of the EGRflow. Using an intake oxygen sensor, the EGR flow may be estimated bycomparing the oxygen content of intake air when the EGR valve is open,to a base level when the EGR valve is closed. As soot builds up, the EGRflow estimate obtained from the intake oxygen sensor may be compared tothe EGR flow estimate obtained from the DP and position sensors (hereinalso referred to as the DPOV system). FIG. 3 further shows how thedifference in EGR flow estimates obtained from the oxygen sensor and DPand position sensors may then be used to ascertain an estimate of sootaccumulation on the EGR valve. By considering changes in the effectivevalve flow area due to soot accumulation, subsequent estimates of EGRflow based on the DP and position sensors may be adjusted based on thedetermined soot accumulation.

FIG. 1 shows a schematic depiction of an example turbocharged enginesystem 100 including a multi-cylinder internal combustion engine 10 andtwin turbochargers 120 and 130, which may be identical. As onenon-limiting example, engine system 100 can be included as part of apropulsion system for a passenger vehicle. While not depicted herein,other engine configurations such as an engine with a single turbochargermay be used without departing from the scope of this disclosure.

Engine system 100 may be controlled at least partially by a controller12 and by input from a vehicle operator 190 via an input device 192. Inthis example, input device 192 includes an accelerator pedal and a pedalposition sensor 194 for generating a proportional pedal position signalPP. Controller 12 may be a microcomputer including the following: amicroprocessor unit, input/output ports, an electronic storage mediumfor executable programs and calibration values (e.g., a read only memorychip), random access memory, keep alive memory, and a data bus. Thestorage medium read-only memory may be programmed with computer readabledata representing non-transitory instructions executable by themicroprocessor for performing the routines described herein as well asother variants that are anticipated but not specifically listed.Controller 12 may be configured to receive information from a pluralityof sensors 165 and to send control signals to a plurality of actuators175 (various examples of which are described herein). Other actuators,such as a variety of additional valves and throttles, may be coupled tovarious locations in engine system 100. Controller 12 may receive inputdata from the various sensors, process the input data, and trigger theactuators in response to the processed input data based on instructionor code programmed therein corresponding to one or more routines.Example control routines are described herein with regard to FIGS. 2-3and 5-6.

Engine system 100 may receive intake air via intake passage 140. Asshown at FIG. 1, intake passage 140 may include an air filter 156 and anair induction system (AIS) throttle 115. The position of AIS throttle115 may be adjusted by the control system via a throttle actuator 117communicatively coupled to controller 12.

At least a portion of the intake air may be directed to a compressor 122of turbocharger 120 via a first branch of the intake passage 140 asindicated at 142 and at least a portion of the intake air may bedirected to a compressor 132 of turbocharger 130 via a second branch ofthe intake passage 140 as indicated at 144. Accordingly, engine system100 includes a low-pressure AIS system (LP AIS) 191 upstream ofcompressors 122 and 132, and a high-pressure AIS system (HP AIS) 193downstream of compressors 122 and 132.

A positive crankcase ventilation (PCV) conduit 198 (e.g., push-sidepipe) may couple a crankcase (not shown) to the second branch 144 of theintake passage such that gases in the crankcase may be vented in acontrolled manner from the crankcase. Further, evaporative emissionsfrom a fuel vapor canister (not shown) may be vented into the intakepassage through a fuel vapor purge conduit 195 coupling the fuel vaporcanister to the second branch 144 of the intake passage.

The first portion of the total intake air can be compressed viacompressor 122 where it may be supplied to intake manifold 160 viaintake air passage 146. Thus, intake passages 142 and 146 form a firstbranch of the engine's air intake system. Similarly, a second portion ofthe total intake air can be compressed via compressor 132 where it maybe supplied to intake manifold 160 via intake air passage 148. Thus,intake passages 144 and 148 form a second branch of the engine's airintake system. As shown at FIG. 1, intake air from intake passages 146and 148 can be recombined via a common intake passage 149 beforereaching intake manifold 160, where the intake air may be provided tothe engine. In some examples, intake manifold 160 may include an intakemanifold pressure sensor 182 for estimating a manifold pressure (MAP)and/or an intake manifold temperature sensor 183 for estimating amanifold air temperature (MCT), each communicating with controller 12.In the depicted example, intake passage 149 also includes a charge aircooler (CAC) 154 and a throttle 158. The position of throttle 158 may beadjusted by the control system via a throttle actuator 157communicatively coupled to controller 12. As shown, throttle 158 may bearranged in intake passage 149 downstream of CAC 154, and may beconfigured to adjust the flow of an intake gas stream entering engine10.

As shown at FIG. 1, a compressor bypass valve (CBV) 152 may be arrangedin CBV passage 150 and a CBV 155 may be arranged in CBV passage 151. Inone example, CBVs 152 and 155 may be electronic pneumatic CBVs (EPCBVs).CBVs 152 and 155 may be controlled to enable release of pressure in theintake system when the engine is boosted. An upstream end of CBV passage150 may be coupled with intake passage 148 downstream of compressor 132,and a downstream end of CBV passage 150 may be coupled with intakepassage 144 upstream of compressor 132. Similarly, an upstream end of aCBV passage 151 may be coupled with intake passage 146 downstream ofcompressor 122, and a downstream end of CBV passage 151 may be coupledwith intake passage 142 upstream of compressor 122. Depending on aposition of each CBV, air compressed by the corresponding compressor maybe recirculated into the intake passage upstream of the compressor(e.g., intake passage 144 for compressor 132 and intake passage 142 forcompressor 122). For example, CBV 152 may open to recirculate compressedair upstream of compressor 132 and/or CBV 155 may open to recirculatecompressed air upstream of compressor 122 to release pressure in theintake system during selected conditions to reduce the effects ofcompressor surge loading. CBVs 155 and 152 may be either actively orpassively controlled by the control system.

As shown, a compressor inlet pressure (CIP) sensor 196 is arranged inthe intake passage 142 and a HP AIS pressure sensor 169 is arranged inintake passage 149. However, in other anticipated embodiments, sensors196 and 169 may be arranged at other locations within the LP AIS and HPAIS, respectively. Among other functions, CIP sensor 196 may be used todetermine a pressure downstream of an EGR valve 121.

Engine 10 may include a plurality of cylinders 14. In the depictedexample, engine 10 includes six cylinders arrange in a V-configuration.Specifically, the six cylinders are arranged on two banks 13 and 15,with each bank including three cylinders. In alternate examples, engine10 can include two or more cylinders such as 3, 4, 5, 8, 10 or morecylinders. These various cylinders can be equally divided and arrangedin alternate configurations, such as V, in-line, boxed, etc. Eachcylinder 14 may be configured with a fuel injector 166. In the depictedexample, fuel injector 166 is a direct in-cylinder injector. However, inother examples, fuel injector 166 can be configured as a port based fuelinjector.

Intake air supplied to each cylinder 14 (herein, also referred to ascombustion chamber 14) via common intake passage 149 may be used forfuel combustion and products of combustion may then be exhausted viabank-specific exhaust passages. In the depicted example, a first bank 13of cylinders of engine 10 can exhaust products of combustion via acommon exhaust passage 17 and a second bank 15 of cylinders can exhaustproducts of combustion via a common exhaust passage 19.

The position of intake and exhaust valves of each cylinder 14 may beregulated via hydraulically actuated lifters coupled to valve pushrods,or via mechanical buckets in which cam lobes are used. In this example,at least the intake valves of each cylinder 14 may be controlled by camactuation using a cam actuation system. Specifically, the intake valvecam actuation system 25 may include one or more cams and may utilizevariable cam timing or lift for intake and/or exhaust valves. Inalternative embodiments, the intake valves may be controlled by electricvalve actuation. Similarly, the exhaust valves may be controlled by camactuation systems or electric valve actuation. In still anotheralternative embodiment, the cams may not be adjustable.

Products of combustion that are exhausted by engine 10 via exhaustpassage 17 can be directed through exhaust turbine 124 of turbocharger120, which in turn can provide mechanical work to compressor 122 viashaft 126 in order to provide compression to the intake air.Alternatively, some or all of the exhaust gases flowing through exhaustpassage 17 can bypass turbine 124 via turbine bypass passage 123 ascontrolled by wastegate 128. The position of wastegate 128 may becontrolled by an actuator (not shown) as directed by controller 12. Asone non-limiting example, controller 12 can adjust the position of thewastegate 128 via pneumatic actuator controlled by a solenoid valve. Forexample, the solenoid valve may receive a signal for facilitating theactuation of wastegate 128 via the pneumatic actuator based on thedifference in air pressures between intake passage 142 arranged upstreamof compressor 122 and intake passage 149 arranged downstream ofcompressor 122. In other examples, other suitable approaches other thana solenoid valve may be used for actuating wastegate 128.

Similarly, products of combustion that are exhausted by engine 10 viaexhaust passage 19 can be directed through exhaust turbine 134 ofturbocharger 130, which in turn can provide mechanical work tocompressor 132 via shaft 136 in order to provide compression to intakeair flowing through the second branch of the engine's intake system.Alternatively, some or all of the exhaust gases flowing through exhaustpassage 19 can bypass turbine 134 via turbine bypass passage 133 ascontrolled by wastegate 138. The position of wastegate 138 may becontrolled by an actuator (not shown) as directed by controller 12. Asone non-limiting example, controller 12 can adjust the position ofwastegate 138 via a solenoid valve controlling a pneumatic actuator. Forexample, the solenoid valve may receive a signal for facilitating theactuation of wastegate 138 via the pneumatic actuator based on thedifference in air pressures between intake passage 144 arranged upstreamof compressor 132 and intake passage 149 arranged downstream ofcompressor 132. In other examples, other suitable approaches other thana solenoid valve may be used for actuating wastegate 138.

In some examples, exhaust turbines 124 and 134 may be configured asvariable geometry turbines, wherein controller 12 may adjust theposition of the turbine impeller blades (or vanes) to vary the level ofenergy that is obtained from the exhaust gas flow and imparted to theirrespective compressor. Alternatively, exhaust turbines 124 and 134 maybe configured as variable nozzle turbines, wherein controller 12 mayadjust the position of the turbine nozzle to vary the level of energythat is obtained from the exhaust gas flow and imparted to theirrespective compressor. For example, the control system can be configuredto independently vary the vane or nozzle position of the exhaust gasturbines 124 and 134 via respective actuators.

Products of combustion exhausted by the cylinders via exhaust passage 19may be directed to the atmosphere via exhaust passage 180 downstream ofturbine 134, while combustion products exhausted via exhaust passage 17may be directed to the atmosphere via exhaust passage 170 downstream ofturbine 124. Exhaust passages 170 and 180 may include one or moreexhaust after-treatment devices, such as a catalyst, and one or moreexhaust gas sensors. For example, as shown at FIG. 1, exhaust passage170 may include an emission control device 129 arranged downstream ofthe turbine 124, and exhaust passage 180 may include an emission controldevice 127 arranged downstream of the turbine 134. Emission controldevices 127 and 129 may be selective catalytic reduction (SCR) devices,three way catalysts (TWC), NO_(x) traps, various other emission controldevices, or combinations thereof. Further, in some embodiments, duringoperation of the engine 10, emission control devices 127 and 129 may beperiodically regenerated by operating at least one cylinder of theengine within a particular air/fuel ratio, for example.

Engine system 100 may further include one or more exhaust gasrecirculation (EGR) systems for recirculating at least a portion ofexhaust gas from the exhaust manifold to the intake manifold. These mayinclude one or more high-pressure EGR systems for proving high pressureEGR (HP EGR) and one or more low-pressure EGR-loops for providing lowpressure EGR (LP EGR). In one example, HP EGR may be provided in theabsence of boost provided by turbochargers 120, 130, while LP EGR may beprovided in the presence of turbocharger boost and/or when exhaust gastemperature is above a threshold. In still other examples, both HP EGRand LP EGR may be provided simultaneously.

In the depicted example, engine system 100 may include a low-pressure(LP) EGR system 108. LP EGR system 108 routes a desired portion ofexhaust gas from exhaust passage 170 to intake passage 142. In thedepicted embodiment, EGR is routed in an EGR passage 197 from downstreamof turbine 124 to intake passage 142 at a mixing point located upstreamof compressor 122. The amount of EGR provided to intake passage 142 maybe varied by the controller 12 via EGR valve 121 coupled in the LP EGRsystem 108. In the example embodiment shown at FIG. 1, LP EGR system 108includes an EGR cooler 113 positioned upstream of EGR valve 121. EGRcooler 113 may reject heat from the recirculated exhaust gas to enginecoolant, for example. The LP EGR system may include a differentialpressure over valve (differential pressure or delta Pressure or DP)sensor 125. In one example, an EGR flow rate may be estimated based onthe DPOV system which includes the DP sensor 125 that detects a pressuredifference between an upstream region of the EGR valve 121 and adownstream region of EGR valve 121. EGR flow rate (e.g., LP EGR flowrate) determined by the DPOV system may be further based on an EGRtemperature detected by an EGR temperature sensor 135 located downstreamof EGR valve 121 and an area of EGR valve opening detected by an EGRvalve lift sensor 131. In another example, EGR flow rate may bedetermined based on outputs from an EGR measurement system that includesan intake oxygen sensor (herein referred to as IAO2 sensor) 168, massair flow sensor (not shown), manifold absolute pressure (MAP) sensor 182and manifold temperature sensor 183. In some examples, both the EGRmeasurement systems (that is, the DPOV system including differentialpressure sensor 125 and the EGR measurement system including intakeoxygen sensor 168) may be used to determine, monitor and adjust EGR flowrate.

In an alternate embodiment, the engine system may include a second LPEGR system (not shown) that routes a desired portion of exhaust gas fromexhaust passage 180 to intake passage 144. In another alternateembodiment, the engine system may include both the LP EGR systems (onerouting exhaust gas from exhaust passage 180 to intake passage 144, andanother routing exhaust gas from exhaust passage 170 to intake passage142) described above.

In a further embodiment while not shown in FIG. 1, the engine system 100may also include a high pressure EGR system which may route a desiredportion of exhaust gas from common exhaust passage 17, upstream of theturbine 124, to intake manifold 160, downstream of intake throttle 158.

EGR valve 121 may include a body and stem (not shown), where said stemis movable within the body of the EGR valve 121 such that the opening ofthe EGR valve 121 may be adjusted based on the relative position of thestem and body. The EGR valve 121 may be configured to adjust an amountand/or rate of exhaust gas diverted through the EGR passage to achieve adesired EGR dilution percentage of the intake charge entering theengine, where an intake charge with a higher EGR dilution percentageincludes a higher proportion of recirculated exhaust gas to air than anintake charge with a lower EGR dilution percentage. In addition to theposition of the EGR valve, it will be appreciated that AIS throttleposition of the AIS throttle 115, and other actuators may also affectthe EGR dilution percentage of the intake charge. As an example, AISthrottle position may increase the pressure drop over the LP EGR system,allowing more flow of LP EGR into the intake system. As a result, thismay increase the EGR dilution percentage, whereas less LP EGR flow intothe intake system may decrease the EGR dilution percentage (e.g.,percentage EGR). Accordingly, EGR dilution of the intake charge may becontrolled via control of one or more of EGR valve position and AISthrottle position among other parameters. Thus, adjusting the EGR valves121 and/or the AIS throttle 115 may adjust and EGR flow amount (or rate)and subsequently a percentage EGR in the mass air flow (e.g., air chargeentering the intake manifold).

The engine 10 may further include one or more oxygen sensors positionedin the common intake passage 149. As such, the one or more oxygensensors may be referred to as intake oxygen sensors. In the depictedembodiment, an intake oxygen sensor 168 is positioned upstream ofthrottle 158 and downstream of CAC 154. However, in other embodiments,intake oxygen sensor 168 may be arranged at another location alongintake passage 149, such as upstream of the CAC 154. Intake oxygensensor (IAO2) 168 may be a variable voltage (VVs) oxygen sensor or anysuitable sensor for providing an indication of the oxygen concentrationand EGR concentration of the intake charge air (e.g., air flowingthrough the common intake passage 149). In one example, the intakeoxygen sensors 168 may be an intake oxygen sensor including a heatedelement as the measuring element. During operation, a pumping current ofthe intake oxygen sensor may be indicative of an amount of oxygen in thegas flow.

A pressure sensor 172 may be positioned alongside the oxygen sensor forestimating an intake pressure at which an output of the oxygen sensor isreceived. Since the output of the oxygen sensor is influenced by theintake pressure, a reference oxygen sensor output may be learned at areference intake pressure. In one example, the reference intake pressureis a throttle inlet pressure (TIP) where pressure sensor 172 is a TIPsensor. In alternate examples, the reference intake pressure is amanifold pressure (MAP) as sensed by MAP sensor 182.

Engine system 100 may include various sensors 165, in addition to thosementioned above. As shown in FIG. 1, common intake passage 149 mayinclude a throttle inlet temperature sensor 173 for estimating athrottle air temperature (TCT). Further, while not depicted herein, eachof intake passages 142 and 144 may include a mass air flow sensor oralternatively the mass air flow sensor can be located in common duct140.

Humidity sensor 189 may be included in only one of the parallel intakepassages. As shown in FIG. 1, the humidity sensor 189 is positioned inthe intake passage 142 (e.g., non PCV and non-purge bank of the intakepassage), upstream of the CAC 154 and an outlet of the LP EGR passage197 into the intake passage 142 (e.g., junction between the LP EGRpassage 197 and the intake passage 142 where LP EGR enters the intakepassage 142). Humidity sensor 189 may be configured to estimate arelative humidity of the intake air. In one embodiment, humidity sensor189 is a UEGO sensor configured to estimate the relative humidity of theintake air based on the output of the sensor at one or more voltages.Since purge air and PCV air can confound the results of the humiditysensor, the purge port and PCV port are positioned in a distinct intakepassage from the humidity sensor.

Intake oxygen sensor 168 may be used for estimating an intake oxygenconcentration and inferring an amount of EGR flow through the enginebased on a change in the intake oxygen concentration upon opening of theEGR valve 121. Specifically, a change in the output of the sensor uponopening the EGR valve 121 is compared to a reference point where thesensor is operating with no EGR (the zero point). Based on the change(e.g., decrease) in oxygen amount from the time of operating with noEGR, an EGR flow currently provided to the engine can be calculated. Forexample, upon applying a reference voltage (Vs) to the sensor, a pumpingcurrent (Ip) is output by the sensor. The change in oxygen concentrationmay be proportional to the change in pumping current (delta Ip) outputby the sensor in the presence of EGR relative to sensor output in theabsence of EGR (the zero point). Based on a deviation of the estimatedEGR flow from the expected (or target) EGR flow, further EGR control maybe performed.

A zero point estimation of the intake oxygen sensor 168 may be performedduring idle conditions where intake pressure fluctuations are minimaland when no PCV or purge air is ingested into the low pressure inductionsystem. In addition, the idle adaptation may be performed periodically,such as at every first idle following an engine start, to compensate forthe effect of sensor aging and part-to-part variability on the sensoroutput.

A zero point estimation of the intake oxygen sensor may alternatively beperformed during engine non-fueling conditions, such as during adeceleration fuel shut off (DFSO). By performing the adaptation duringDFSO conditions, in addition to reduced noise factors such as thoseachieved during idle adaptation, sensor reading variations due to EGRvalve leakage can be reduced.

Thus, the system of FIG. 1 provides for a system for an engine,comprising: a turbocharger with an intake compressor and an exhaustturbine, a low-pressure exhaust gas recirculation (EGR) passage coupledbetween an exhaust passage downstream of the exhaust turbine and theintake passage upstream of the intake compressor, the low-pressure EGRpassage including an EGR valve and DPOV system for measuring EGR flow,an intake oxygen sensor disposed in an intake of the engine downstreamfrom the low-pressure EGR passage, and a controller withcomputer-readable instructions for indicating flow-area degradation ofthe EGR valve based on a difference between a first EGR flow estimatebased on an output of the DP sensor and a second EGR flow estimate basedon an output of the intake oxygen sensor during engine operation withpurge disabled, boost disabled, and mass air flow below a thresholdlevel. The intake oxygen sensor may be further positioned in an intakemanifold of the engine and the computer-readable instructions furtherinclude instructions for adjusting a third EGR flow estimate, the thirdEGR flow estimate based on the output of the DP sensor during engineoperation when one or more of purge is enabled, boost is enabled, andmass air flow is greater than the threshold level, based on thedifference between the first EGR flow estimate and the second EGR flowestimate.

FIG. 2 shows a flow chart of a method 200 for estimating EGR flow in alow-pressure EGR system using an intake oxygen sensor (such as IAO2 168shown in FIG. 1) and/or a DP sensor (e.g., DP sensor 125 shown inFIG. 1) of a DPOV system based on engine operating conditions.Instructions for carrying out method 200 may be stored in a memory of anengine controller such as controller 12 shown in FIG. 1. Further, method200 may be executed by the controller. The controller may estimate theEGR mass flow rate using a DP sensor which measures the pressuredifferential across the EGR valve and a valve position sensor (e.g.,such as EGR valve lift sensor 131). However, as explained earlier, assoot accumulates on the EGR valve, the EGR mass flow rate estimate usingthe DPOV method described above may become increasingly inaccurate.Thus, under some conditions an IAO2 sensor may be used to estimate anEGR mass flow rate to provide an EGR flow rate estimate with increasedaccuracy. The IAO2 sensor may also be used to determine an estimate ofsoot accumulation on the EGR valve. Because there may be significanterror in the IAO2 sensor's measurements under certain engine operatingconditions (e.g. boosted engine operation, intake mass air flow over athreshold) the IAO2 sensor may not be used at all times. Thus, method200 additionally comprises determining when to use the IAO2 sensor toestimate the EGR mass flow rate. Method 200 further involves comparingan EGR flow estimate obtained from the IAO2 sensor with that from a DPOVsystem. This may provide an accurate EGR flow estimate from the DPOVsystem with improved accuracy of the DPOV system input of EGR valve flowarea. Method 200 begins at 202 and the controller (e.g. controller 12)estimates and/or measures engine operating conditions based on feedbackfrom a plurality of sensors. Engine operating conditions may includeengine temperature, engine speed and load, intake mass air flow,manifold pressure, a position of the EGR valve, a position of a purgevalve, etc.

Method 200 proceeds to 204 where the controller determines if EGR is on,based on feedback from a position sensor (e.g. EGR valve lift sensor131) about the position of the EGR valve. In another example, thecontroller may determine that EGR is on based on an EGR flow beinggreater than zero. In this way, EGR flow may be on if EGR is flowingthrough the low-pressure EGR passage (e.g. EGR passage 197) from theexhaust passage to the intake passage. If the controller determines thatthe EGR valve is closed and EGR is off, then method 200 continues to 206where the controller operates an intake air oxygen (IAO2) sensor (e.g.IAO2 sensor 168 shown in FIG. 1) to measure the intake air oxygen level.The IAO2 sensor is configured to apply a base reference voltage (V₀)across a pumping electrode pair which pumps oxygen out of or into aninternal cavity and generates a pumping current that may be used toinfer the oxygen level (i.e., the partial pressure of O₂) in the intakeair flow. In one embodiment, the IAO2 sensor may be a variable voltage(VVs) oxygen sensor. If at 204 the controller determines that EGR is on,then the controller proceeds to 208 to determine if purge and boost areoff.

At 208 and 214, the controller determines if purge and boost are off andif the intake mass airflow is below a threshold, to determine whether touse outputs from the IAO2 sensor or the DP and EGR valve positionsensors to estimate the EGR flow. At 208, the controller determines ifpurge and boost are off. When the engine is not boosted, the IAO2 sensormay provide a more accurate estimate of the EGR flow rate than the DPsensor using the DPOV method. However, if the EGR estimation isperformed using outputs from the IAO2 during conditions when fuelcanister purge and/or crankcase ventilation is enabled (e.g., PCV flowis enabled), an output of the IAO2 sensor may be corrupted by theadditional hydrocarbons flowing to the sensor. Thus, under boostedengine conditions, the DP sensor may provide a more accurate estimate ofthe EGR flow. The IAO2 sensor output may be corrupted primarily duringboosted conditions due to ingested hydrocarbons reacting with ambientoxygen at the sensing element of the intake sensor. This reduces the(local) oxygen concentration read by the sensor. Since the output of thesensor and the change in oxygen concentration is used to infer an EGRdilution of intake aircharge, the reduced oxygen concentration read bythe intake oxygen sensor in the presence of purge air and/or PCV may beincorrectly interpreted as additional diluent. Thus, if the controllerdetermines that either purge or boost are on at 208, then method 200proceeds to 210 and the controller estimates EGR mass flow rate usingthe DPOV system comprising a delta pressure (DP) sensor (e.g. deltapressure sensor 125) and position sensor (e.g. EGR valve lift sensor131). The EGR mass flow rate may be proportional to the cross-sectionalarea of the EGR valve opening and the differential pressure across theEGR valve (as determined from the DP sensor). An estimate of the crosssectional area (e.g., flow area) of the EGR valve opening may becomputed from the displacement of the EGR valve (e.g., valve lift)provided by the position sensor, a known cross-sectional flow area ofthe EGR valve, and a valve lift correction factor. The knowncross-sectional flow area of the EGR valve is a standard cross-sectionalarea of the valve perpendicular to the direction of EGR flow through thevalve. The valve lift correction factor may increase the accuracy in theestimate of the flow area by taking into account thermal effects on theexpansion of the EGR valve. For example, this thermal compensationmethod may include using a determined difference between the EGR valvestem and body temperatures to estimate a change in the known (orexpected) EGR valve flow area, as explained in greater detail below withreference to FIGS. 5 & 6. The estimate of the cross sectional area ofthe EGR valve opening (e.g., the EGR valve flow area perpendicular tothe direction of flow through the valve) together with the estimatedpressure differential across the EGR valve as provided by the DP sensormay be used to estimate the EGR mass flow rate (referred to herein asthe DPOV method). Once the EGR mass flow rate is estimated, thecontroller then adjusts engine operation based on the estimated EGR massflow rate at 212. As an example, if the estimated EGR mass flow rate isless than a desired rate, the controller may command the EGR valve toopen further and allow more exhaust gases to be recirculated to theintake passage (e.g. common intake passage 149). The desired EGR ratemay be determined by the controller based on engine operating conditionssuch as engine load and engine speed.

If at 208 the controller determines that purge and boost are off, thenthe controller determines if the intake mass air flow is greater than athreshold at 214 based on feedback from a mass air flow sensor in theair intake passage of the engine. Estimating EGR flow based on theoutput of the IAO2 may include multiplying the output of the sensor by afactor based on the mass air flow in order to convert the output to anEGR flow rate or flow percentage. Thus, in one example, the thresholdmass air flow may be based on a mass air flow at which an error in theEGR flow estimate using the IAO2 sensor increases above an acceptablelevel (or increases above an error in estimating EGR flow using the DPOVmethod). When calculating the EGR mass estimate using the IAO2 sensorand mass air flow, the EGR mass estimate is influenced by the accuracyof the mass air flow measurement. The relative air flow error on the EGRmass flow estimate may be lower at lower air mass flow rates. Thethreshold air mass flow rate may be chosen such that the air mass flowerror is small compared to the EGR mass flow estimate.

If the intake mass air flow is below the threshold at 214, then the IAO2sensor may provide a more accurate estimate of the EGR mass flow ratethan the DPOV system. As such, the controller may proceed to 216 andestimate the EGR flow rate using the IAO2 sensor. As described in FIG.1, the intake oxygen sensor may apply a reference voltage which maygenerate an output in the form of a pumping current (Ip) that may beused to determine the oxygen concentration of the surrounding gas in thecommon intake passage 149. The controller may then estimate an EGRconcentration in the intake air based on a change in the intake oxygenconcentration when the EGR valve is open and EGR is on (e.g. EGR valve121) to a reference point where the EGR valve is closed and EGR is off.In other words, based on the change (e.g., decrease) in oxygenconcentration determined when EGR is operating, to a time of operationwith no EGR, the controller may estimate the EGR flow. Subsequently, thecontroller may adjust engine operation based on the estimated EGR massflow rate at 218. As an example, if the estimated EGR mass flow rate isless than a desired rate, the controller may command the EGR valve toopen further and allow more exhaust gases to be recirculated to theintake passage (e.g. common intake passage 149). Additionally, thecontroller may route more exhaust gas through EGR passage 197. Thedesired EGR rate may be determined by the controller based on engineoperating conditions such as engine load and engine speed.

Alternatively, if the intake mass air flow is above the threshold,estimations of the EGR mass flow rate using the IAO2 sensor may bedegraded. As discussed above, in order to estimate the EGR mass flow,the controller may convert EGR concentration as estimated by the IAO2sensor to an EGR mass flow rate by multiplying the intake air mass airflow rate by the ratio of the concentrations of EGR to intake air. Inother words, errors in the EGR flow estimate may be increased inmagnitude when multiplying by higher mass air flow values. In this way,estimates of the EGR flow may become increasingly inaccurate at higherair flow levels. If intake mass air flow levels are above the thresholdat 214, then estimation of the EGR mass flow rate may be more accurateusing the DPOV method than the IAO2 sensor. Thus, if the controllerdetermines at 214 that the intake mass air flow is above the threshold,then the controller may estimate the EGR mass flow rate using the DPOVsystem at 210. Subsequently at 212, the controller may adjust engineoperation based on the estimated EGR mass flow rate. As an example, ifthe estimated EGR mass flow rate is less than the desired rate, thecontroller may command the EGR valve to open further and allow moreexhaust gases to be recirculated to the intake passage (e.g. commonintake passage 149). The desired rate may be determined based on engineoperating parameters such as engine load, engine temperature, etc. asdescribed in greater detail in FIG. 1.

As described above, the controller may estimate the EGR mass flow rateusing the IAO2 sensor so long as purge and boost are off, and the intakemass air flow is below a threshold. Otherwise, the DPOV system may beused to estimate the EGR mass flow rate. Thus, in one example, undernon-boosted engine conditions, the IAO2 may provide a more accurateestimate of the EGR mass flow rate than the DPOV system. Once the EGRmass flow rate has been estimated either by using the IAO2 sensor or theDPOV system, the controller then adjusts engine operation based on theestimated EGR flow rate at 218 and 212 respectively. In one embodiment,the controller may adjust engine operation by increasing or decreasingthe amount of EGR by way of opening or closing the EGR valve to matchthe desired EGR flow rate. If the estimated EGR mass flow rate is lessthan the desired rate, then the controller may command the EGR valve toopen further to allow for more EGR. On the other hand if the estimatedEGR is higher than the desired EGR, the controller may command the EGRvalve to close an amount thereby reducing the EGR flow.

Returning to method 200, after adjusting engine operation based on theestimated EGR flow rate using the IAO2 sensor at 218, the controllerthen continues to 220 and determines if it is time for EGR valve arealearning. EGR valve area learning may be a method for increasing theaccuracy of EGR mass flow rate estimates when it may not be desirable touse the IAO2 sensor, such as under boosted engine operating conditions,and the DPOV system may be used instead. As described in greater detailin FIG. 3, the valve area learning may involve correcting an estimate ofthe EGR valve flow area by comparing two EGR flow estimates, oneobtained from outputs of the IAO2 sensor, and the other obtained fromoutputs of the DPOV system. Since valve area learning requires ameasurement from the IAO2 sensor, it may only occur during non-boostedengine conditions when purge is also disabled, and when intake mass airflow is below a threshold. The controller may determine when valve arealearning occurs based on pre-set timing intervals between instances ofvalve area learning. In one embodiment, the time interval between valvelearning instances may be a number of engine cycles. Thus, if a pre-setnumber of engine cycles have passed since the most recent instance ofEGR valve area learning, then the controller may determine that it istime to initiate another valve area learning sequence. As stated abovehowever, valve area learning may only occur so long as the engine isoperating under non-boosted conditions and intake mass air flow is belowa threshold. In another embodiment, the time interval between valve arealearning instances may be a duration of engine use. In a furtherembodiment, the time interval may be a period of time. Thus, it isimportant to note that during engine operation, EGR valve area learningmay happen many times, each instance producing a correction factor forthe estimation of the effective flow area through the EGR valve, whereeach valve error correction may update an earlier correction determinedfrom a previous valve area learning instance.

If the controller determines that it is time for EGR valve area (e.g.,EGR valve flow area) learning, then method 200 continues on to 224 andthe controller estimates the EGR mass flow rate using the DPOV system(via the DPOV method described above) and compares it to the EGR flowrate estimate obtained from the IAO2 sensor. The difference in the twoestimates may be used to correct expected EGR valve flow area estimates(based on the EGR valve lift sensor and any other lift corrections) andindicate an amount of soot accumulation on the EGR valve, as elaboratedbelow with reference to FIG. 3. Specifically, EGR flow rate estimatesobtained using the IAO2 sensor may be less than EGR flow estimates usingthe DPOV system due to soot accumulation on the EGR valve. The soot mayblock parts the EGR valve opening and reduce the effective flow areathrough the valve. The reduced flow area may result in lower EGR massflow rates at the common intake passage 149 where the oxygen sensor maybe positioned. The difference in the two estimates of the EGR mass flowrate may then be used by the controller to determine a corrected EGRvalve flow area. For a greater description of how the controllerdetermines the corrected EGR valve flow area, see FIG. 3. If thedifference between the two estimates of EGR mass flow is greater than athreshold, then the controller may indicate that soot has accumulated onthe EGR valve.

If at 220 the controller determines that a pre-set time interval has notbeen reached since the most recent instance of valve area learning, thenthe controller continues engine operation without performing EGR valveoffset learning at 222. A correction factor for the flow area throughthe EGR valve from a previous valve offset learning instance may then beused when using the DPOV method to determine EGR flow for enginecontrol.

Method 200 may further entail a method for an engine, comprisingindicating soot accumulation on an exhaust gas recirculation (EGR) valvebased on a difference in EGR flow estimated, during a first conditionwhen the engine is not boosted, with an intake oxygen sensor and with apressure sensor coupled across the EGR valve. The difference in EGR flowis a difference between a first EGR flow estimated based on an output ofthe intake oxygen sensor during the first condition and a second EGRflow estimated with the pressure sensor across the EGR valve during thefirst condition, where the pressure sensor is a differential pressureover valve (DP sensor). The method may further comprise estimating thesecond EGR flow based on an output of the DP sensor and a flow area ofthe EGR valve, where the flow area of the EGR valve is estimated basedon a known cross-section of the EGR valve and an EGR valve positionbased on an output of an EGR valve position sensor. The first conditionfurther includes when purge is disabled and mass air flow to the engineis less than a threshold level. Thus, method 200 further comprisesadjusting engine operation based on EGR flow estimated with the intakeoxygen sensor and not the pressure sensor coupled across the EGR valvewhen the engine is not boosted, purge is disabled, and mass air flow tothe engine is below a threshold level. Method 200 further comprisesdetermining a change in EGR valve flow area based on the difference inEGR flow, an expected EGR valve flow area and a first EGR flow estimatedwith the intake oxygen sensor during the first condition, the expectedEGR valve flow area based on an output of an EGR valve position sensorand an EGR valve lift correction, the EGR valve lift correction learnedduring an EGR valve end stop and thermal compensation learning routine.

An indication of soot accumulation on the EGR valve may be given basedon the change in the effective EGR valve flow area. Method 200 maycomprise indicating soot accumulation on the EGR valve based on thechange in EGR valve flow area increasing above threshold level. Inanother example, the method may further comprise indicating sootaccumulation on the EGR valve based on a rate of change in the change inEGR valve flow area increasing above a threshold rate.

Method 200 may further entail determining a corrected EGR valve flowarea based on the determined change in EGR valve flow area and theexpected EGR valve flow area and during a second condition when EGR flowis estimated with the DP sensor, estimating EGR flow based on the outputof the DP sensor and the corrected EGR valve flow area. The secondcondition includes one or more of when the engine is boosted, when purgeis enabled, and when mass air flow to the engine is greater than athreshold level.

Turning now to FIG. 3, a method 300 for indicating soot accumulation onan EGR valve and determining a corrected EGR valve flow area is shown.Instructions for carrying out method 300 may be stored in a memory of anengine controller such as controller 12 shown in FIG. 1. Further, method300 may be executed by the controller.

Method 300 may continue from step 224 in method 200. As such, method 300begins at 302 where the controller estimates EGR mass flow rate with theIAO2 sensor and the EGR valve DPOV system. As described above withreference to FIG. 2, the controller may compute the EGR mass flow (e.g.,EGR flow rate or flow percentage) by comparing the intake oxygenconcentrations estimated by the oxygen sensor when the EGR valve is openand EGR is on (e.g. EGR valve 121) to a reference point where the EGRvalve is closed and EGR is off. The controller may estimate the EGR massflow rate using the DPOV system based on estimates of the pressuredifferential across the EGR valve, and the flow area through the valveopening. A DP sensor may provide the pressure differential over the EGRvalve. A position sensor coupled to the EGR valve may provide thedisplacement (e.g., lift) of the EGR valve. The flow area through theEGR valve can then be estimated based on the position of the EGR valve,a known cross-sectional flow area of the valve, and a compensation(e.g., correction) based on the expansion and/or contraction of the EGRvalve due to thermal effects (e.g., due to a temperature differentialbetween the EGR valve stem and body), as described in greater detailbelow with reference to FIGS. 5 & 6.

Once the controller estimates the EGR mass flow rate with the IAO2sensor and DPOV system at 302, it then adjusts the DPOV EGR estimate fora delay to the location of the IAO2 sensor at 304. Said another way, thedelay may be a measurement delay due to the different locations of theIAO2 sensor and EGR valve relative to one another. As described in FIG.2, the DPOV EGR flow rate estimate may be different than the IAO2 sensorEGR flow rate estimate. This difference may be at least partially causedbecause the IAO2 sensor and DPOV system are measuring different exhaustgases, or it could be the result of a systematic error in one of themeasuring systems. If soot accumulation on the EGR valve decreases theeffective flow area of the EGR valve, the DPOV system may systematicallyoverestimate the EGR flow rate. An object of method 300 is to estimate asystematic error in the DPOV measuring system and learn a valve areacorrection factor that may be used to provide more accurate estimates ofthe EGR valve flow area and therefore EGR flow estimates. Thus, in orderto judge the accuracy of the DPOV system by comparing its EGR mass flowrate estimate to that of the IAO2 sensor, it is important that the IAO2sensor and the DPOV system measure the same exhaust gases at the samepoint in time. As can be seen in FIG. 1, exhaust gas must travel adistance from the EGR passage 197 to the common intake passage 149.Thus, it takes time for exhaust gas exiting the EGR valve in the EGRpassage to reach the IAO2 sensor located in the common intake passage.Measuring the same exhaust gases at both the EGR valve and the IAO2sensor may require a time delay adjustment to the DPOV and/or IAO2 EGRestimates. Thus the controller may delay the DPOV flow rate estimate tothe IAO2 EGR estimate to account for the time it takes the exhaust gasto travel from the EGR valve to the intake oxygen sensor. In this way,the resulting EGR flow estimates from the DP sensor (of the DPOV system)and the IAO2 sensor may reflect an EGR flow estimate for the same EGRgas.

As one example, the controller may take simultaneous measurements fromboth the DPOV system and the IAO2 sensor. In one embodiment, thecontroller may then apply a pre-set time delay correction factor to theIAO2 sensor measurement. The delay correction factor may be based on anestimated time for exhaust gases to travel from the EGR valve to thecommon intake passage which may be based on airflow rates. In anotherembodiment, the controller may apply a pre-set time delay correctionfactor to the DPOV system measurement. The delay correction factor maybe based on an estimated time for exhaust gases to travel from the EGRvalve to the common intake passage which may be based on airflow rates.For example, in a transient operation, the EGR valve and the deltapressure across the EGR valve (DP) are changing. The EGR mass flow ratecalculated at a given instance needs to be compared with the EGR massflow rate measured by the IAO2 sensor. To be able to compare the sameEGR flows, the DPOV measurement of the EGR flow rate is delayed by thetime it takes for EGR measurement in question to reach the IAO2 sensor.

In another example, the controller may delay the EGR flow rate estimateusing the IAO2 sensor from the DPOV system EGR flow rate estimate. Inother words, the measurement of EGR at the IAO2 sensor may occur at apoint in time slightly after the DP and position sensor measurements atthe EGR valve. The EGR flow rate estimation made by the IAO2 sensor maybe slightly delayed to account for the time it takes the exhaust gasesto travel from the EGR valve to the intake exhaust sensor. Thus, thecontroller may determine two estimations of the EGR mass flow rate thatare separated in time, but based on measurements of the same exhaustgases. The exact timing between the two measurements may be determinedby the controller based on the intake air flow rate, pressure (boostpressure) and temperature. Thus, for faster flow rates, the duration ofthe delay between the two measurements may be less than for slower flowrates.

In another example, the controller may record several measurements fromboth the DPOV system and the IAO2 sensor over a period of time of EGRoperation under non-boosted engine conditions. The controller may thendetermine the amount of time it takes exhaust gases to travel to theintake oxygen sensor from the EGR valve based on the estimated airflowrate. Subsequently, the controller may determine which measurementstaken from the DPOV system and the IAO2 sensor correspond to the samemeasured exhaust gases. For a given DPOV measurement, the controller mayfirst determine the time at which that measurement was taken, add thetime it takes the exhaust gas to travel to the intake oxygen sensor, andthen determine which IAO2 sensor measurement occurred at the later time.The controller may then use those two measurements to obtain twoestimates of the EGR mass flow.

Moving forward to 306, the controller may determine an EGR flow rateerror from the difference between the IAO2 and DPOV EGR flow rateestimates. An assumption in step 306 may be that the EGR flow rateestimate given by the IAO2 sensor is more accurate than the flow rategiven by the DPOV system. Thus, the IAO2 EGR flow rate estimate istreated as the actual estimated EGR flow rate. As explained above, thisis a reasonable assumption to make under non-boosted engine conditionswhen intake air flow rates are below a threshold flow rate. The EGR flowrate estimate obtained from the DPOV system may be have reduced accuracydue to soot accumulation on the EGR valve. In this case, the EGR flowrate estimate obtained from the IAO2 sensor may be less than thatobtained from the DPOV system. The EGR flow rate error would be the IAO2EGR flow rate estimate subtracted from the DPOV EGR flow rate estimate.After estimating the EGR flow rate error, the controller determines thechange in the EGR valve flow area based on the EGR flow error at 310.The controller may divide the EGR flow rate error by the IAO2 EGR flowrate estimate, giving a percent error in the DPOV EGR flow rateestimate. Then, multiplying the percent error in the DPOV EGR flow rateestimate by the EGR valve flow area (e.g., an expected or known EGRvalve flow area) may give an estimate of the change in the EGR valveflow area due to soot accumulation. The EGR valve flow area may be thesame EGR valve flow area estimate used in the DPOV EGR flow ratecalculation. In another embodiment, the controller may estimate the EGRvalve flow area after making the estimate of the EGR flow rate. Based onthe change in EGR valve flow area, the controller may then determine thecorrected EGR valve flow area at 312. The corrected EGR valve flow areamay be the difference between the estimated flow area obtained from theEGR valve position sensor (e.g., EGR valve lift sensor), and theestimated change in valve flow area. As explained in FIG. 2, sootaccumulation on the EGR valve may reduce the flow area through the valveand cause the EGR flow rate as estimated by the IAO2 sensor to be lessthan that estimated by the DPOV system. Thus, the change in valve flowarea may be directly related to the amount of soot accumulation on theEGR valve and may be used to indicate an amount of soot on the EGRvalve.

Once the EGR valve area learning has occurred at 312, method 300 thenproceeds to 314, and the controller uses the corrected EGR valve flowarea for subsequent DPOV EGR flow rate estimates and adjusts EGR basedon the corrected EGR flow rate estimate. If the controller uses thecorrected EGR valve flow area to estimate the EGR flow rate, and thatflow rate is different than the desired flow rate, then the controllermay adjust engine operation to match the EGR flow rate to the desiredrate. In one example, if the EGR flow rate is less than the desiredrate, the controller may command the EGR valve to open further and allowmore exhaust gases to be recirculated to the intake passage. Thecontroller may additionally increase the amount of exhaust gasesentering EGR passage 197. The desired flow rate may be based on engineoperating parameters such as engine load, engine speed, enginetemperature, exhaust gas temperature, etc. as measured by a plurality ofengine sensors. Thus, when the controller uses outputs from the DPOVsystem to estimate the EGR flow rate, it may use the corrected EGR valveflow area to do so. As noted in FIG. 2, the controller may continuallyupdate the corrected EGR valve flow area due to new estimates of the EGRflow rate based on measurements from the IAO2 sensor. As engineoperation continues, so may soot accumulation on the EGR valve. If sootaccumulation reaches high enough levels, then the accuracy of the DPOVEGR flow rate may reduce further. The difference between the desired EGRflow and the actual EGR flow may also be larger at greater soot levelsdue to the inaccurate EGR flow rate estimates. At 316, the controllerdetermines if the EGR valve flow area error is greater than a threshold.In one example, the EGR valve flow area error may be a differencebetween the actual EGR valve flow area (based on the EGR valve positionsensor output) and the corrected EGR valve flow area. In another examplethe EGR valve flow area error may be based on the EGR flow rate error.If the controller determines that the EGR valve error is greater thanthe threshold, then it continues to 320 to indicate EGR valvedegradation and/or initiate a valve cleaning routine.

In one embodiment, the threshold EGR valve error may be based on thedifference between the EGR valve area error and the most recentlydetermined EGR valve area error. As explained in FIG. 2, the controllermay continually update the EGR valve area error based on new estimatesof the EGR flow rate from the IAO2 sensor. Thus, if the EGR valve areaerror is greater than the most recently determined EGR valve area errorby more than a threshold, the controller may indicate EGR valvedegradation and/or initiate valve cleaning routine. In anotherembodiment, the threshold EGR valve error may be based on the differencebetween the EGR valve area error and a raw estimate of the EGR valveflow area based only on the position of the EGR valve and a known areaof the valve without using the EGR valve flow error correction. The rawestimate of the EGR valve flow area could be a first estimate of thevalve flow area that was made before any soot accumulation and valvearea learning. The controller may archive the raw estimate for theduration of engine operation. The raw estimate may also be generatedeach time the DPOV system is used to estimate the valve flow area, andthen the EGR valve flow error correction may be applied after. In otherwords, the flow area may first be determined by the position of the EGRvalve and a known area of the valve. Then, the EGR valve error may beapplied to correct the estimate of the flow area. The controller maycompare the EGR valve area estimate before valve error correction to theEGR valve area estimate after valve error correction. If the differencebetween the two valve area estimates is greater than a threshold, thenthe controller may continue to 320. The threshold may also be thought ofas a threshold amount of soot accumulation on the EGR valve, since thechange in EGR valve area may be directly related to the sootaccumulation on the EGR valve. In another embodiment, the threshold maybe based on the rate of change of the change in the EGR valve areaestimate. If the rate of change in valve flow area as estimated by theEGR error increases above a threshold, then the controller may proceedto 320.

At 320, the controller may indicate EGR valve degradation. Theindication may be given to the user via a user feedback display such asa light switch on the dashboard. The controller may additionally oralternatively initiate a valve cleaning routine which may be used toreduce the amount of soot on the EGR valve. In still another example,indicating EGR valve degradation may include setting a diagnostic code.

If at 316, the controller determines that the EGR valve area error isnot greater than a threshold, then the controller continues to 318 andcontinues EGR valve operation without indicating EGR valve degradationand/or initiating a valve cleaning routine.

Method 300 involves estimating a change in the EGR valve flow area dueto soot accumulation. In one example, the determined change in EGR flowarea may be used to correct the EGR flow rate estimated using the DPOVmethod. The method may further include indicating EGR valve degradationand/or initiating an EGR valve cleaning routine if soot accumulation hasreached a threshold level, as determined based on an EGR valve areaerror, a change in the EGR valve flow area, and/or a rate of change inthe EGR valve flow area and/or the EGR flow rate error.

FIG. 3 includes a method for an engine, comprising: during selectedconditions, comparing a first exhaust gas recirculation (EGR) flowestimated based on an output of an intake oxygen sensor with a secondEGR flow estimated based on a pressure difference across an EGR valve,and indicating soot build-up on the EGR valve based on the comparison.Comparing the first EGR flow with the second EGR flow includes learninga flow area error of the EGR valve based on a difference between thefirst EGR flow and second EGR flow. Indicating soot build-up on the EGRvalve includes indicating degradation of the EGR valve due to soot basedon the learned flow area error increasing above a threshold. The methodmay further comprise during subsequent engine operation when EGR flow isestimated based on the pressure difference across the EGR valve,adjusting the EGR flow estimate based on the learned flow area error.The pressure difference across the EGR valve may be measured via adifferential pressure over valve (DP) sensor coupled across the EGRvalve. Indicating soot build-up on the EGR valve includes one or more ofsetting a diagnostic code, initiating an EGR valve cleaning routine, andalerting a vehicle operator that the EGR valve is degraded and needsservicing. The selected conditions may include when the engine is notboosted, purge is disabled, and when mass air flow is less than athreshold level. Indicating soot accumulation includes one or more ofinitiating a cleaning routine, alerting a vehicle operator that the EGRvalve is degraded, and setting a diagnostic code.

Determining the adjusted valve flow area is further based on a secondchange in flow area due to EGR valve soot accumulation. The method maycomprise determining the second change in flow area based on adifference in EGR flow estimated, during a first condition when theengine is not boosted, with an intake oxygen sensor and with a pressuresensor coupled across the EGR valve. Determining the second change inflow area is further based on an expected EGR valve flow area and afirst EGR flow estimated with the intake oxygen sensor during the firstcondition, the expected EGR valve flow area based on an output of an EGRvalve position sensor and an EGR valve lift correction, the EGR valvelift correction learned during an EGR valve end stop and thermalcompensation learning routine. The method may further compriseindicating soot accumulation on the EGR valve based on the based on thesecond change in flow area increasing above threshold level.

Moving on to FIG. 4, a graph illustrating how EGR flow may be estimatedunder varying engine conditions is shown. Specifically, a graph 400shows changes in EGR flow as measured by an EGR valve DPOV system atplot 402 and as measured by an IAO2 sensor at plot 404. Graph 400 alsoshows an EGR valve flow rate error at plot 406, an estimated amount ofsoot buildup on an EGR valve at 408, an intake mass air flow rate atplot 410, a boost condition of the engine at plot 412, and a purgecondition at plot 414. The EGR valve flow rate error is essentially thedifference between the EGR flow rate estimates from the DPOV system andIAO2 sensor as described in greater detail in the method in FIG. 3.Thus, the EGR valve flow rate error is the error in the DPOV EGR flowrate estimate based on the difference between the EGR flow rateestimates from the IAO2 sensor and the DPOV system. The soot buildup maybe inferred from the EGR flow rate error as described by the method inFIG. 3. More specifically, the EGR flow rate error may be used toestimate an EGR flow area error since the EGR flow rate error may becaused by a change in the EGR valve flow area due to soot accumulation.The EGR flow area error may then be used to infer an amount of sootaccumulation. Intake mass air flow may be measured by a mass air flowsensor. The operational status of boost and purge may be regulated by acontroller (e.g. controller 12). Thus, the controller may determinepurge levels based on the position of a purge valve in the purge passage(e.g., fuel vapor purge conduit 195). The controller may determine boostlevels via the operational status of the turbines, compressors, or bycommands sent to the turbocharger.

As described above with reference to FIGS. 2 & 3, both a DPOV system andan IAO2 sensor may be used to estimate the EGR mass flow rate in aturbocharged engine. Both the DPOV system and the IAO2 sensor may onlybe used under certain operating conditions to estimate EGR mass flowrate estimates, due to sensor corruption under conditions like boosting,purge, etc., as elaborated in FIG. 2. Thus, EGR flow rate estimates fromthe DPOV system and the IAO2 sensor may have varying levels of accuracydepending on the engine operating conditions. For example, when theintake mass air flow is above a threshold, the DPOV system may providemore accurate EGR flow rate estimates than the IAO2 sensor. However,under non-boosted engine conditions when purge is disabled, and when theintake mass air flow is below a threshold, the IAO2 sensor measurementsmay produce more accurate estimates of the EGR mass flow rate than theDPOV system. The controller may determine whether the EGR flow rateestimate from the DPOV system or the IAO2 sensor is more accurate basedon engine operating conditions. In another embodiment, the controllermay use a combination of the EGR flow rate estimates to estimate the EGRflow rate. Thus, if the controller determines that purge and boost aredisabled, and the intake mass airflow is below a threshold, then thecontroller may use the IAO2 sensor EGR estimates to correct and increasethe accuracy of the DPOV system EGR flow rate estimates. Thus, bycapitalizing on IAO2 measurements when the engine is operating underselect operating conditions (e.g. non-boosted, purge disabled, lowintake mass airflow conditions), the estimates of the EGR mass flow rateunder all vehicle operating conditions may be improved. This isespecially helpful because as soot accumulates on the EGR valve, theDPOV system EGR flow rate estimates may become increasingly inaccuratedue to the decreased in valve flow area caused by the soot. Thus, byusing the IAO2 sensor as a reference point, the DPOV system EGR rateestimates may be corrected to account for the decreased EGR valve flowarea due to soot accumulation. If soot accumulation reaches a criticalthreshold, then a valve cleaning routine may be initiated, or anindication of soot buildup may be reported to the vehicle operator.

Starting before time t₁, boost is on (plot 412) (e.g., the engine isboosted), while purge is off (plot 414), and the intake mass air flow isbelow a threshold T₁ (plot 410). Since boost is on, EGR flow estimatesvia the IAO2 sensor (plot 404) are not being taken, as can be seen fromthe absence of plot 404 before time t₁. The EGR valve flow rate errormay be at a first level E₁ (plot 406). The first level E₁ may be an EGRvalve flow rate error estimated from a previous valve area learningevent. The soot buildup may be at a first level S₁ corresponding to theEGR valve flow area error level E₁. At time t₁, boost is turned off, butthe intake mass air flow spikes above the threshold T₁. Thus, thecontroller continues to use the DPOV system to estimate the EGR massflow rate without considering outputs from the IAO2 sensor. Becausemeasurements from the IAO2 sensor are not being taken, no new valve arealearning may occur, and thus there may be no new estimates of the EGRvalve flow rate error or soot buildup. Thus, the EGR valve flow areaerror and the soot buildup remain the same before and after t₁ at levelsE₁ and S₁, respectively.

At time t₂ purge is turned on, and the intake mass air flow drops backbelow the threshold T₁. The IAO2 sensor measurements continue to beunused by the controller, and thus EGR valve flow rate error and sootbuildup estimates remain unchanged. At time t3, the intake mass air flowincreases above the threshold T₁, and boost is turned on. Purge alsoremains on. The controller continues to neglect IAO2 sensormeasurements, and the soot buildup estimate stays at first level S₁ andEGR valve flow area error remains at E₁. It is important to note thatthe three engine operating parameters (intake mass air flow, boost, andpurge) may be on or above a threshold level in any combination. So longas either boost or purge is on, or the intake mass air flow is above T₁,however, the IAO2 sensor measurements will remain unused by thecontroller, and the estimates for soot buildup and EGR valve flow rateerror will remain unchanged.

At time t₄, the intake mass air flow drops below threshold T₁, and bothpurge and boost are turned off. Thus, at time t₄ the controller mayestimate EGR flow using the IAO2 sensor output (at plot 404). At thispoint, EGR mass flow rate estimates are obtained from both the IAO2sensor and the DPOV system. The controller may compare the two EGR flowestimates and estimate an EGR valve flow rate error as described in themethod of FIG. 3. Because the IAO2 sensor may not have been used toestimate EGR flow before time t₄, an amount of soot may have accumulatedon the EGR valve, as seen in the spike on plot 408 from first level S₁to a higher second level S2. The soot buildup may cause inaccuracies inthe DPOV system EGR flow rate estimates, specifically in the estimationof the EGR valve flow area. Accordingly, in plot 406 at time t₄, the EGRvalve flow rate error increases from a first error E₁ to a higher seconderror, E₂.

From time t₄ to time t₅, boost and purge remain off, and the intake massair flow stays below T₁. EGR mass flow rate estimates based on the IAO2sensor output continue to be taken during this time, but they maydiverge from the DPOV EGR mass flow rate estimates. This may be due tosoot accumulation on the EGR valve increasing from time t₄ to time t₅.Thus, as more and more soot accumulates on the EGR valve, the differencebetween the estimations of the EGR mass flow rate between the DPOVsystem and the IAO2 sensor may increase. As seen in plots 406 and 408from time t₄ to time t₅, soot accumulation steadily increases, and sodoes the EGR valve flow rate error. Then at time t₅, the controllercorrects the EGR valve area estimate as described in the method of FIG.3. Due to the correction, the EGR mass flow rate as estimated by theDPOV system becomes more accurate, and more similar to that of theestimate obtained from the IAO2 sensor. Thus at time t₅ the EGR valveflow rate error decreases from a higher level E₄ to a lower levelsimilar to that of E₁. Meanwhile, the soot buildup continues toincrease. At time t₅, although soot buildup continues to increase, thecontroller uses the IAO2 sensor measurements as a reference point tocorrect estimates of the EGR valve flow rate using the DPOV system.Specifically, the controller may use the difference between the IAO2sensor and the DPOV system EGR flow rate estimates to infer an error inthe EGR valve flow area estimates. Since the EGR valve flow area is usedto estimate the DPOV system EGR flow rate, errors in the EGR valve flowarea due to soot accumulation may cause errors in the DPOV system EGRflow rate estimates. Thus, disparities between IAO2 sensor and DPOVsystem EGR flow rate estimates may be attributed to errors in the EGRvalve flow area estimates as a result of soot accumulation on the EGRvalve. Accordingly, the EGR valve flow rate error may be used to inferan EGR valve flow area error. By accounting for changes in the EGR valveflow area due to soot accumulation on the EGR valve, the accuracy of theDPOV EGR flow rate estimates are increased.

Moving forward in time to time t₆, the intake mass air flow increasesabove the threshold T₁. As seen at plot 404, the controller ceases touse outputs from the IAO2 sensor for EGR flow rate estimates at time t₆.The DPOV system continues to take measurements (plot 402), and thecontroller uses the corrected EGR valve flow area estimated from the EGRflow rate error at time t₅ to estimate the EGR mass flow rate. Thus, theEGR valve flow rate error remains constant after time t₆ since no newIAO2 sensor measurements are used to compare to the DPOV systemestimates. Similarly, after time t₆, soot may continue to accumulate onthe EGR valve, but without accurate IAO2 measurements, the controllermay be unable to measure and/or estimate soot levels. As such, as seenat plot 408, soot levels as estimated by the controller, remain constantafter time t₆.

At time t₇, the intake mass air flow decreases below the threshold T₁,while boost and purge remain off. Thus, the controller may estimate EGRflow based on outputs of the IAO2 sensor at time t₇. The EGR mass flowrate estimates from the IAO2 sensor may be less than the estimates fromthe DPOV system. Accordingly, the EGR valve flow rate error increasesfrom a level similar to that of E₁ to a higher level E₃ due thedisparity in the two estimates of the EGR mass flow rate. Sootaccumulation as estimated by the controller increases from a lower levelS₃ to a higher level S₄ due to the difference between the IAO2 sensorand DPOV sensor EGR flow estimates. From time t₇ to time t₈, the EGRmass flow rate estimates from the DPOV system and IAO2 sensor divergedue to increased soot accumulation on the EGR valve, and thus more errorin the EGR valve area estimate from the DPOV system. The EGR valve flowrate error steadily increases, until at time t₈, the controller correctsthe EGR valve flow area estimate as described in the method of FIG. 3,just as it did at time t₅. With the corrected EGR valve area, the DPOVsystem EGR mass flow rate estimate more closely matches the IAO2 sensorestimate. The EGR valve flow rate error decreases to a lower levelsimilar to that of E₁. Meanwhile soot accumulation continues toincrease. Thus, at time t₅ and time t₈ the controller initiates valvearea learning, and corrects the EGR valve area estimate from the DPOVsystem, so that the EGR mass flow rate estimate from the DPOV systemmore closely matches the estimate from the IAO2 sensor.

Moving on to time t₉, soot accumulation reaches a threshold T₂. Asdescribed in the method in FIG. 3, the controller may indicate to avehicle operator that the EGR valve is degraded at time t₉, or it mayinitiate a valve cleaning routine in response to the soot accumulationreaching the threshold T2. If the controller indicates EGR valvedegradation to a vehicle operator, soot accumulation may continue toincrease after time t₉. However, if the controller initiates a valvecleaning routine, then soot may be removed from the EGR valve, and sootlevels may drop to a lower level S₅, similar to that of S₁. After timet₉, the intake mass air flow remains below the threshold T₁, and boostand purge remain off. Thus, IAO2 sensor measurements continue to be usedto estimate the EGR mass flow rate, and soot accumulates on the EGRvalve. Accordingly, the EGR valve flow rate error increases, and the EGRmass flow rate estimates from the DPOV system and IAO2 sensor may differfrom one another.

Graph 400 shows how the controller may estimate EGR mass flow ratesdepending on engine operating conditions. In one embodiment, thecontroller may estimate EGR mass flow using only the DPOV system whenone or more of the intake mass air flow is above a threshold, boost ison, or purge is enabled. Under conditions when the intake mass air flowis below a threshold, boost is off and purge is disabled, the controllermay use the IAO2 sensor to estimate the EGR mass flow rate due to theincreased accuracy of the IAO2 sensor under these conditions. In otherembodiments, the controller may estimate EGR flow using both the IAO2sensor and the EGR valve DPOV system; however, the controller may thendecide which estimate to use based on a relative accuracy of eachmeasurement, the relative accuracy based on engine operating conditionssuch as boost level, purge level, and/or mass air flow. The controllermay compare the DPOV system EGR mass flow estimates to those of the IAO2sensor to judge an amount of error in the DPOV system EGR flowestimates. The controller may then correct the DPOV system EGR flow rateestimates based on the IAO2 sensor estimates. The error in the DPOVsystem EGR mass flow estimates may increase during engine use, as sootmay accumulate on the EGR valve. Soot accumulation may affect theestimates of the EGR valve flow area and thus the EGR flow estimates aswell. By using the IAO2 sensor measurements as a comparative referencepoint, the DPOV system EGR mass flow rate estimates may be corrected bytaking into account the decreased EGR valve flow area as a result ofsoot accumulation. Further, when soot accumulation reaches a thresholdsoot level, the controller may signal that the EGR valve has beendegraded and/or or it may initiate a valve cleaning routine to clearsoot off the EGR valve.

Moving on to FIG. 5, a method 500 for learning changes in EGR valve flowarea due to changes in a temperature difference between an EGR valvestem and body is provided. As a temperature difference between the bodyand the stem of the EGR valve increases, the cross-sectional flow areaof the EGR valve may change due to thermal expansion or contraction,thereby increasing the error in the EGR valve flow area and thus theresulting EGR flow estimate using the DPOV method. More accurateestimates of the EGR valve flow area may increase the accuracy ofestimates of the EGR flow rate using the DPOV system. Thus, method 500may provide a means for more accurately estimating the EGR flow rateusing the DPOV system. As described earlier in FIGS. 2 and 3, the DPOVsystem may estimate an EGR flow rate based on a pressure differenceacross the EGR valve (e.g. EGR valve 121), and a flow area through theEGR valve. The EGR valve flow area may be estimated based on theposition of the EGR valve (as determined by a lift sensor) and a knowncross-sectional flow area of the valve. Method 500 provides a correctionfactor for estimating the EGR valve flow area that is based on thermalexpansion of the EGR valve. Instructions for carrying out method 500 maybe stored in a memory of an engine controller such as controller 12shown in FIG. 1. Further, method 500 may be executed by the controller.

Method 500 begins at 502 where the controller estimates and/or measuresengine operating parameters. Engine operating parameters may beestimated based on feedback from a plurality of sensors and may include:engine temperature, engine speed and load, intake mass air flow,manifold pressure, etc.

The controller then continues to 504 and determines if it is time forEGR valve thermal compensation learning. Valve thermal compensationlearning may involve estimating a change in the EGR valve flow areabased on a change in the difference between the temperature of a stemand body of the EGR valve, as described further below. The controllermay determine the timing of the thermal compensation learning based onhow much time has elapsed since the most recent thermal compensationlearning event. Thus, the controller may initiate valve thermalcompensation learning after a pre-set amount of time has elapsed sincethe most recent thermal compensation learning event. The pre-set amountof time may be a number of engine cycles, duration of engine use, or aperiod of time. Thus, if the pre-set amount of time has not elapsedsince the most recent thermal compensation learning event, then thecontroller may determine that thermal compensation is not needed and mayproceed to 506. At 506, the controller may use a previously determinedEGR valve area correction from a prior thermal compensation learningevent for DPOV EGR estimates. This previously determined EGR valve areacorrection may then be used in the methods of FIGS. 2-3 to moreaccurately estimate EGR flow using the DPOV method.

If the pre-set amount of time has elapsed since the most recent thermalcompensation learning event, then the controller may determine that itis time for thermal compensation learning and proceed to 508. At 508,the controller estimates the difference in temperature between the stemand body of the EGR valve based on the EGR temperature. Specifically,the temperature difference between the stem and body of the EGR valvemay be stored in the memory of the controller as a function of EGRtemperature. The relationship between the difference in EGR valve stemand body temperature and the EGR temperature may be pre-set based onfactory testing. The EGR temperature may be estimated by a temperaturesensor (e.g., EGR temperature sensor 135) either upstream or downstreamof the EGR valve. The temperature registered by the temperature sensormay be adjusted depending on the position of the temperature sensorrelative to the EGR valve. Exhaust gas may cool as it travels through anexhaust passage (e.g., EGR passage 197), and thus the temperatureregistered by a sensor downstream of the EGR valve may be lower than theactual temperature of the exhaust gas as it passes through the EGRvalve. Conversely, an upstream temperature sensor may register anexhaust gas temperature warmer than that at the EGR valve. The amount ofchange in the temperature of the exhaust gas from the position of theEGR valve to the temperature sensor may be pre-determined by factorytesting and based on EGR flow. Thus, the temperature of the exhaust gasregistered by the temperature sensor may be modified to represent thetemperature of the exhaust gas at the EGR valve. With estimates of theEGR temperature, the controller may estimate the difference intemperature between the EGR valve stem and body using a knownrelationship between the EGR temperature, and EGR valve stem and bodytemperature difference. The resulting difference in temperature may thenbe modified based on EGR flow. In this way, the temperature differentbetween the EGR valve stem and body may be based on EGR temperature andEGR flow.

Subsequently, at 510, the controller may determine the difference intemperature between the EGR valve stem and body at an EGR valve closingposition (ΔT_(ESL)) corresponding to substantially the same EGRtemperature as used to determine the ΔT_(vlv) at 508. More specifically,at 508 the difference between the stem and body temperature of the EGRvalve is determined at a current EGR temperature. Whenever the EGR valvecloses (e.g., closes completely such that no EGR is flowing though theEGR passage), the controller may store the temperature differencebetween the stem and body of the EGR valve as a function of the EGRtemperature, as described further below with reference to FIG. 6.Therefore, the controller may retrieve (e.g., look up) a body and stemtemperature difference corresponding to the same EGR temperature atwhich the ΔT_(vlv) was estimated at in 508.

Method 500 may then proceed to 512 and the controller may determine thechange in EGR valve flow area based on the difference between ΔT_(vlv)and ΔT_(ESL) and a thermal expansion coefficient. Specifically, thecontroller may multiply the difference between ΔT_(vlv) and ΔT_(ESL) bya thermal expansion coefficient to give an estimate of the change in theEGR valve flow area. In one example, the thermal expansion coefficientmay be predetermined based on the type of material comprising the EGRvalve.

After determining the change in the EGR valve flow area, the controllermay continue to 514 and determine a corrected EGR valve flow area foruse in subsequent DPOV EGR flow estimates. Thus, the corrected EGR valveflow area may be determined based on the change in the EGR valve flowarea. Specifically, as described earlier in FIG. 2, the EGR valve flowarea may be determined from the position of the EGR valve (as determinedby a lift sensor of the EGR valve) and a known cross-sectional flow areaof the EGR valve. However, due to the thermal expansion of the EGRvalve, the EGR valve flow area may be different than the flow areadetermined by using the position and known area of the EGR valve. Thus,using the change in valve flow area due to thermal expansion of the EGRvalve, the accuracy of the estimation of the EGR valve flow area may beincreased and may more closely match the actual flow area through theEGR valve and thus the effective EGR flow rate through the valve. Whendetermining the EGR valve flow area therefore, the controller makeconsider both the effects of thermal expansion of the valve and theamount of soot accumulation on the valve. Thus, the controller mayestimate a first EGR valve area correction factor based on the thermalexpansion of the EGR valve. The controller may also determine a secondEGR valve area correction factor based on soot deposition on the EGRvalve. By incorporating the two EGR valve area correction factors, thecontroller may determine a total EGR valve flow area correction factor.Thus, the accuracy of the EGR valve area estimates may be increased andmay subsequently be used to provide more accurate DPOV EGR flowestimates as described earlier in FIG. 2. Specifically, the EGR valvearea estimate and the pressure differential across the EGR valve asmeasured by the DP sensor may be used to infer an EGR rate.

In this way, a method for an engine may comprise adjusting an exhaustgas recirculation (EGR) valve based on an estimate of EGR flow, the EGRflow estimated based on a pressure difference across the EGR valve andan adjusted valve flow area, the adjusted valve flow area based on afirst temperature difference between a stem and body of the EGR valve.The pressure difference across the EGR valve may be estimated with apressure sensor across the EGR valve, wherein the pressure sensor is adifferential pressure over valve (DP) sensor, and wherein the adjustedvalve flow area is further based on a known cross-section of the EGRvalve and an EGR valve position, the EGR valve positioned measured withan EGR valve position sensor (e.g., such as a lift sensor). The adjustedvalve flow area is adjusted from a known cross-sectional flow area ofthe EGR valve and an output of an EGR valve position sensor. The methodmay further comprise determining the adjusted valve flow area based on afirst change in flow area based on the first temperature differencebetween the stem and body of the EGR valve and a thermal expansioncoefficient of the EGR valve. The method may further include: at eachclosing event of the EGR valve, determining a second temperaturedifference between the stem and body of the EGR valve at an EGR valveclosing position and storing the determined second temperaturedifference at the EGR valve closing position in a memory of acontroller. The first change in flow area is further based on adifference between the first temperature difference between the stem andbody of the EGR valve and the second temperature difference between thestem and body of the EGR valve at the EGR valve closing position. Themethod may further comprise estimating the first temperature differencebased on a temperature and flow rate of EGR gas flowing through the EGRvalve.

In another example, a method for an engine may further comprisedetermining an EGR valve lift correction based on a change in atemperature difference of a stem and body of the EGR valve between whenthe valve is open and closed, where the temperature difference of thestem and body of the EGR valve is based on an EGR temperature measuredproximate to the EGR valve and EGR flow.

In another example, a method for an engine comprises: estimating anexhaust gas recirculation (EGR) flow based on a pressure differenceacross an EGR valve and a total valve flow area, learning a first valveflow area correction factor based on a first temperature differencebetween a stem and body of the EGR valve, and adjusting the total valveflow area based on the first learned valve flow area correction factor.Learning the first valve flow area correction factor includes storingthe learned first valve flow area correction factor in a memory of acontroller and repeating the learning the first valve flow areacorrection factor after a duration, the duration including one or moreof a duration of engine operation and a number of engine cycles.Learning the first valve flow area correction factor includes estimatingthe first temperature difference between the stem and body of the EGRvalve based on EGR flow and a temperature of exhaust flowing through theEGR valve. Learning the first valve flow area correction factor includesestimating the first temperature difference between the stem and body ofthe EGR valve based on EGR flow and a temperature of exhaust gas flowingthrough the EGR valve. Learning the first valve flow area correctionfactor includes multiplying the difference between the first temperaturedifference and the second temperature difference by a thermal expansioncoefficient of the EGR valve, where the thermal expansion coefficient isa coefficient of thermal expansion of valve lift per degree oftemperature difference between the stem and body of the EGR valve. Themethod for an engine further comprising learning a second valve flowarea correction factor based on a difference between a first EGR flowestimated based on an output of an intake oxygen sensor and a second EGRflow estimated based on the pressure difference across the EGR valveduring engine operation with purge disabled, boost disabled, and massair flow below a threshold level and further comprising adjusting thetotal valve flow area based on the first learned valve flow areacorrection factor and the second valve flow area correction factor.Estimating the EGR flow includes estimating the EGR flow based on thepressure difference across the EGR valve and the total valve flow areaduring a first condition when one or more of engine purge and boost areon, and when intake mass airflow is above a threshold. The methodfurther comprises estimating the EGR flow based on an output of anintake oxygen sensor and not the pressure difference across the EGRvalve during a second condition when engine purge and boost are off andthe intake mass airflow is below the threshold.

In another example, a system for an engine comprises: a turbochargerwith an intake compressor and an exhaust turbine, a low-pressure exhaustgas recirculation (EGR) passage coupled between an exhaust passagedownstream of the exhaust turbine and the intake passage upstream of theintake compressor, the low-pressure EGR passage including an EGR valveand DP sensor for estimating EGR flow, an intake oxygen sensor disposedin an intake of the engine downstream from the low-pressure EGR passage,and a controller with computer-readable instructions for adjusting theEGR valve based on the EGR flow estimated based on an output of the DPsensor and an adjusted valve flow area, the adjusted valve flow areabased on a first temperature difference between a stem and body of theEGR valve and a second temperature difference between the stem and bodyof the EGR valve at a closing position of the EGR valve. The intakeoxygen sensor is further positioned in an intake manifold of the engineand wherein the adjusted valve flow area is further based on adifference between a first EGR flow estimate based on an output of theDP sensor and a second EGR flow estimate based on an output of theintake oxygen sensor during engine operation when boost and purge aredisabled and mass air flow is below a threshold level. The engine systemmay further comprise a temperature sensor positioned proximate to theEGR valve within the low-pressure EGR passage and wherein the first andsecond temperature differences are based on an output of the temperaturesensor and EGR flow.

Moving on to FIG. 6, a method 600 for estimating the difference intemperature between the EGR valve stem and body at a valve closingposition is shown (e.g., referred to herein as end-stop learning).Method 600 provides a means for estimating a temperature differencebetween the EGR valve stem and body when the EGR valve is closed(ΔT_(ESL)). Thus, as described earlier in the method of FIG. 5, the(ΔT_(ESL)) may be used to improve the accuracy of the estimated changein the EGR valve flow area due to thermal expansion of the EGR valve.

Method 600 begins at 602 and the controller estimates and/or measuresengine operating conditions. Engine operating parameters may beestimated based on feedback from a plurality of sensors and may include:engine temperature, engine speed and load, intake mass air flow,manifold pressure, EGR valve position, etc.

Based on the engine operating conditions, the controller may thendetermine if the EGR valve is closing at 604. Specifically thecontroller may determine if the EGR valve is closing based on theposition of the EGR valve as given by a position sensor (e.g., EGR valvelift sensor 131). In one embodiment, the controller may continuouslymonitor the EGR valve such that it may proceed to 608 at every valveclosing event. If the controller determines that the valve is notclosing then the controller may proceed to 606. In another embodiment,the controller may not proceed to 608 at every valve closing event.Instead, the controller may only proceed to 608 if the EGR valve isclosing and a duration has passed. Otherwise, the controller may proceedto 606. The duration may be a number of valve closing events, timeinterval, number of engine cycles, etc. Thus, even if the controllerdetects that the valve is closing, the controller may instead proceed to606 if the duration has not passed. At 606, the EGR valve position maybe modulated based on a desired EGR flow rate as determined by engineoperating parameters (e.g., engine temperature, exhaust gas temperature,intake mass air flow, etc.)

However, if at 604 the EGR valve is closing and the duration has passed,then the controller may determine the difference in temperature betweenthe EGR valve stem and body based on the EGR temperature as given by atemperature sensor (e.g., EGR temperature sensor 135). For example, thecontroller may look up the temperature difference between the EGR valvestem and body as a function of EGR temperature and/or EGR flow and thenstore that temperature difference as the ΔT_(ESL) When the EGR valvecloses, the delta temperature calculated is used as the value to bestored for ΔT_(ESL). After determining the ΔT_(ESL) at 608, thecontroller may proceed to 610 and store the ΔT_(ESL) and EGR temperaturevalues determined at 608 in the memory of the controller (e.g., in alook-up table). Thus, the ΔT_(ESL) values stored in the controller maybe accessed to compare to ΔT_(VLV) values to determine a thermalexpansion correction to estimates for the EGR valve flow area asdescribed in the method of FIG. 5.

In this way, a method may include estimating an EGR flow rate based onoutputs from a DPOV system and an intake oxygen sensor. Both the DPOVsystem comprising a delta pressure (DP) sensor and an EGR valve positionsensor, and an intake oxygen sensor may be used to give separateestimates of the EGR mass flow rate. Under engine operating conditionswhere purge is disabled, boost is off, and the intake mass air flow isbelow a threshold, the intake oxygen sensor may be used to give anestimate of the EGR mass air flow rate. The estimate of the EGR mass airflow rate determined from the intake oxygen sensor output may then becompared to an EGR flow estimate based on outputs of the DPOV system todetermine an amount of soot buildup on the EGR valve and thereby providean estimate of the EGR mass air flow rate with increased accuracy.

The DPOV system may estimate the EGR mass flow rate based on thepressure differential across the valve as measured by the DP sensor, andthe area of the EGR valve opening (for EGR flow). The area of the EGRvalve opening may be estimated based on the position of the valve asgiven by a position sensor (e.g., EGR valve lift sensor), a knowncross-sectional flow area of the valve, and a thermal expansioncorrection factor which accounts for the expansion of the valve under acurrent EGR temperature. The cross-sectional flow area (e.g., openingfor EGR flow) of the valve may change depending on the temperaturedifference between the stem and body of the valve. Thus, the area of theEGR valve opening may be modified based on a change between thetemperature difference of the stem and body of the EGR valve when theEGR valve is closed and open and a thermal expansion coefficient.

In this way, the technical effect of determining a corrected EGR flowarea based on soot accumulation on the EGR valve (as determined bycomparing EGR flow estimates from the intake oxygen sensor and the DPOVsystem) and thermal expansion or contraction of the EGR valve (asdetermined by the temperature difference between the stem and body ofthe EGR valve) is determining a more accurate EGR flow estimate toincrease the accuracy of EGR control and additional engine control.Additionally, an amount of soot buildup on an EGR valve may be estimatedand used to initiate a valve cleaning routine, or signal to a vehicleoperator if the soot level reaches a threshold. By using the EGR flowrate based on the oxygen sensor as a reference point, the DPOV systemEGR flow rate estimates may have increased accuracy by accounting forthe decreased flow area caused by soot buildup on the EGR valve. Anothertechnical effect is achieved by adjusting the EGR flow rate based on atemperature difference of the stem and body of the EGR valve when thevalve is open and closed. The opening of the valve may change dependingon the temperature difference between the stem and body of the valve.Thus, the area of the EGR valve opening may be modified based on achange between the temperature difference of the stem and body of theEGR valve when the EGR valve is closed and open and a thermal expansioncoefficient. Subsequently, the flow rate of the EGR may be adjusted tomore closely match a desired EGR flow rate, such that the efficiency ofthe engine may be increased.

In another representation, a method for an engine comprises: duringselected conditions, comparing a first exhaust gas recirculation (EGR)flow estimated based on an output of an intake oxygen sensor with asecond EGR flow estimated based on a pressure difference across an EGRvalve; and indicating soot build-up on the EGR valve based on thecomparison.

In yet another representation, a method for an engine comprises: duringselected conditions, learning a flow area error of an EGR valve based ona difference between EGR flow estimates via an intake oxygen sensor andvia a differential pressure over valve (DP) sensor coupled across theEGR valve; and indicating degradation of the EGR valve due to soot basedon the learned flow area. The method further comprises during subsequentengine operation when EGR is estimated with a DP sensor, adjusting theDPOV EGR estimate based on the learned flow area error.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The control methods and routines disclosed herein may be stored asexecutable instructions in non-transitory memory and may be carried outby the control system including the controller in combination with thevarious sensors, actuators, and other engine hardware. The specificroutines described herein may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various actions,operations, and/or functions illustrated may be performed in thesequence illustrated, in parallel, or in some cases omitted. Likewise,the order of processing is not necessarily required to achieve thefeatures and advantages of the example embodiments described herein, butis provided for ease of illustration and description. One or more of theillustrated actions, operations and/or functions may be repeatedlyperformed depending on the particular strategy being used. Further, thedescribed actions, operations and/or functions may graphically representcode to be programmed into non-transitory memory of the computerreadable storage medium in the engine control system, where thedescribed actions are carried out by executing the instructions in asystem including the various engine hardware components in combinationwith the electronic controller.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

The invention claimed is:
 1. A method for an engine, comprising: openingan EGR valve, thereby allowing exhaust gas to flow there through; andindicating soot accumulation on the exhaust gas recirculation (EGR)valve by setting a diagnostic code based on a difference in EGR flowestimated during a first non-boosted engine condition, with an intakeoxygen sensor and with a pressure sensor coupled across the EGR valve.2. The method of claim 1, wherein the difference in EGR flow is adifference between a first EGR flow estimated based on an output of theintake oxygen sensor during the first condition and a second EGR flowestimated with the pressure sensor across the EGR valve during the firstcondition, wherein the pressure sensor is a differential pressure overvalve (DP) sensor, and further comprising estimating the second EGR flowbased on an output of the DP sensor and a flow area of the EGR valve,where the flow area of the EGR valve is estimated based on a knowncross-section of the EGR valve and an EGR valve position based on anoutput of an EGR valve position sensor.
 3. The method of claim 2,further comprising determining a change in EGR valve flow area based onthe difference in EGR flow, an expected EGR valve flow area and thefirst EGR flow estimated with the intake oxygen sensor during the firstcondition, the expected EGR valve flow area based on the output of theEGR valve position sensor and an EGR valve lift correction, the EGRvalve lift correction learned during an EGR valve end stop and thermalcompensation learning routine.
 4. The method of claim 3, furthercomprising determining the EGR valve lift correction based on a changein a temperature difference of a stem and body of the EGR valve betweenwhen the valve is open and closed, where the temperature difference ofthe stem and body of the EGR valve is based on an EGR temperaturemeasured proximate to the EGR valve and EGR flow.
 5. The method of claim3, further comprising indicating soot accumulation on the EGR valvebased on the change in EGR valve flow area increasing above a thresholdlevel.
 6. The method of claim 3, further comprising indicating sootaccumulation on the EGR valve based on a rate of change in the change inEGR valve flow area increasing above a threshold rate.
 7. The method ofclaim 3, further comprising determining a corrected EGR valve flow areabased on the determined change in EGR valve flow area and the expectedEGR valve flow area.
 8. The method of claim 7, further comprising duringa second condition when EGR flow is estimated with the DP sensor,estimating EGR flow based on the output of the DP sensor and thecorrected EGR valve flow area.
 9. The method of claim 8, wherein thesecond condition includes one or more of when the engine is boosted,when fuel canister purge is enabled, and when mass air flow to theengine is greater than a threshold level.
 10. The method of claim 1,wherein the first condition further includes when fuel canister purge isdisabled and mass air flow to the engine is less than a threshold level.11. The method of claim 1, further comprising adjusting engine operationbased on EGR flow estimated with the intake oxygen sensor and not thepressure sensor coupled across the EGR valve when the engine is notboosted, fuel canister purge is disabled, and mass air flow to theengine is below a threshold level.
 12. The method of claim 1, furthercomprising initiating a cleaning routine responsive to the indication ofsoot accumulation.
 13. A method for an engine, comprising: opening anEGR valve, thereby allowing exhaust gas to flow there through; andduring selected conditions: comparing a first exhaust gas recirculation(EGR) flow estimated based on an output of an intake oxygen sensor witha second EGR flow estimated based on a pressure difference across theEGR valve; and indicating soot build-up on the EGR valve based on thecomparison by setting a diagnostic code.
 14. The method of claim 13,wherein comparing the first EGR flow with the second EGR flow includeslearning a flow area error of the EGR valve based on a differencebetween the first EGR flow and second EGR flow.
 15. The method of claim14, wherein indicating soot build-up on the EGR valve includesindicating degradation of the EGR valve due to soot based on the learnedflow area error increasing above a threshold.
 16. The method of claim14, further comprising during subsequent engine operation when EGR flowis estimated based on the pressure difference across the EGR valve,adjusting the EGR flow estimate based on the learned flow area error.17. The method of claim 13, wherein the pressure difference across theEGR valve is measured via a differential pressure over valve (DP) sensorcoupled across the EGR valve.
 18. The method of claim 13, whereinindicating soot build-up on the EGR valve includes one or more ofsetting a diagnostic code, initiating an EGR valve cleaning routine, andalerting a vehicle operator that the EGR valve is degraded and needsservicing and wherein the selected conditions include when the engine isnot boosted, fuel canister purge is disabled, and mass air flow is lessthan a threshold level.
 19. A system for an engine, comprising: aturbocharger with an intake compressor and an exhaust turbine; alow-pressure exhaust gas recirculation (EGR) passage coupled between anexhaust passage downstream of the exhaust turbine and an intake passageupstream of the intake compressor, the low-pressure EGR passageincluding an EGR valve and differential pressure (DP) sensor formeasuring EGR flow; an intake oxygen sensor disposed in an intake of theengine downstream from the low-pressure EGR passage; and a controllerwith computer-readable instructions for indicating flow-area degradationof the EGR valve based on a difference between a first EGR flow estimatebased on an output of the DP sensor and a second EGR flow estimate basedon an output of the intake oxygen sensor during engine operationresponsive to fuel canister purge disabled, boost disabled, and mass airflow below a threshold level.
 20. The system of claim 19, wherein theintake oxygen sensor is further positioned in an intake manifold of theengine and wherein the computer-readable instructions further includeinstructions for adjusting a third EGR flow estimate, the third EGR flowestimate based on the output of the DP sensor during engine operationwhen one or more of fuel canister purge is enabled, boost is enabled,and mass air flow is greater than the threshold level, based on thedifference between the first EGR flow estimate and the second EGR flowestimate.